Numerical Study On The Viscous Flow And Hydrodynamic Forces On A Manoeuvring Ship At Low Speed

With the rapid development of economy,modern ships are developed towards high speed,large scale and specialization,the safety issue of ship navigation is becoming increasingly prominent,and manoeuvrability becomes one of the most important aspects of the ship hydrodynamic performances at design stage.Meanwhile,with the increase of water transportation of cargoes,the navigation density of ships in port and waterway increases year by year.During navigating in some restricted water areas like nearshore,estuary,port and inland waterway,ships often need to manoeuvre such as moving obliquely and turning under low speed condition in order to reduce the risk of collision and grounding accidents.Besides,marine engineering ships such as cable-laying vessels and rescuing ships usually need to carry out certain special manoeuvres at low speed like moving backward,crabbing,turning with small radius,crash back and crash ahead.In order to navigate safely and operate efficiently,deep understanding of ship manoeuvrability at low speed is needed.Therefore,research on modelling and prediction of ship manoeuvring at low speed is of great significance and practical engineering value.In this thesis,numerical studies on the viscous flow and hydrodynamic force of a ship undergoing some typical manoeuvring motions at low speed are carried out by applying CFD method.First,the viscous flow around a ship in oblique motion at low speed is simulated.Then the viscous flow around a ship in turning motion at low speed is simulated.Besides,the viscous flow around a single propeller operating in four quadrants and in oblique motion is simulated,and the viscous flow around a hull-propeller system in crash back and crash ahead is simulated.For the simulation of oblique motion at low speed,a bare hull model of tanker Esso Osaka is taken as the study object.CFD method is applied to predict the hydrodynamic characteristics at drift angles varying from 0° to 180°.In the simulation,RANS method with the SST k-? turbulent model is adopted for the cases of drift angles 0°~30° and 150°~180°,and DES method is employed for cases with highly separated flow under drift angles 40°~150°.Verification and validation studies are conducted for the flows under drift angles 0° and 70°.The vortex structures of the flow at drift angles 0°,30°,50°,70°,90° and 180° are analyzed.The computed results in terms of velocity contour and turbulent kinetic energy contour between DES and RANS are compared.Finally,the influences of drift angle and ship speed on the lateral force and yaw moment are analyzed.For the simulation of turning motion at low speed,the Esso Osaka bare hull model is taken as the study object.The viscous flow around the ship turning with small radius and large drift angle is simulated,and DES is adopted to simulate the unsteady flow,where viscous effect and flow separation are dominant.Verification study relating to grid and time step discretization is carried out,and the uncertainties are quantified.The predicted lateral forces and yaw moments under different drift angles and a certain turning rate are compared with the experimental data to validate the adopted numerical method,and the variation trends of the friction and pressure components of the lateral forces and yaw moments are clarified.The influences of drift angle and turning radius are investigated through systematic computations.The lateral forces and yaw moments,the flow details like the pressure distribution on the hull surface,the vortical structures and the velocity profiles at some typical planes under different drift angles and turning radii are demonstrated.For the simulation of the viscous flow around a single propeller operating in four quadrants and in oblique motion at low speed,the model of a five-bladed propeller,DTMB4381,is selected as the study object.Numerical simulations of the flow around the propeller under forward,crash back,crash ahead,drifting in forward and backward conditions are performed.The RANS simulation with SST k-? turbulent model is conducted for the viscous flow around the propeller drifting forward and backward to predict the hydrodynamic performance of the propeller.The grid dependency study is carried out and the numerical procedure is validated for the forward condition.LES is applied for simulating the propeller flow under the crash ahead and crash back conditions.The grid dependency study is carried out for the crash back condition.The numerical results of thrust and torque are compared with the available experimental data.The effects of propeller load under crash back and crash ahead conditions,and the effects of drift angle and propeller load under drifting forward and backward conditions are investigated.Flow details like pressure contours on the blades,velocity distribution and streamlines at some typical axial planes are presented to analyze the underlying physical mechanism of the flow.For the simulation of hull-propeller interactions during manoeuvring at low speed,the viscous flow around a hull-propeller system in crash back and crash ahead are simulated.Computations are performed for the tanker KVLCC2 model appended with a propeller and a rudder.The numerical method is validated by comparing the computed self-propulsion point at four forward speeds with experimental data.LES is employed for the simulation under crash back and crash ahead conditions.The influence of propeller load on the hull-propeller interaction is analyzed.The time histories and the corresponding spectra of the thrust and torque under different advance ratios are compared.The pressure distributions on the propeller and the hull-rudder system,the axial velocity contours on several cross planes and longitudinal planes near the stern are examined to reveal the mechanism of hull-propeller interaction.In this thesis,numerical simulations of the viscous flow around ships in various manoeuvring motions at low speed are performed by using the CFD methods.The cases studied include a ship in oblique motion and turning motion;a single propeller under forward,drifting forward and backward,crash back and crash ahead conditions,as well as the hull-propeller interactions in crash back and crash ahead.The flow details and the hydrodynamic forces acting on the hull and propeller obtained by the CFD computations are helpful for better understanding of the flow mechanism during manoeuvring at low speed,which is of great significance in both theory and practice for the research on ship manoeuvring and control at low speed.